Add the given label to the property. The labels of a failing sub-property are displayed when it fails.

ofTestable testable

Signature: testable:'Testable -> PropertyType parameters: 'Testable

Turns a testable type into a property. Testables are unit, boolean, Lazy testables, Gen testables, functions
from a type for which a generator is know to a testable, tuples up to 6 tuple containing testables, and lists
containing testables.

Fails the property if it does not complete within t milliseconds. Note that the called property gets a
cancel signal, but whether it responds to that is up to the property; the execution may not actually stop.